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number of poles on a 3phase motor?

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  • #31
    I was going to make a comment about the number of "poles" in a 3 phase as compared to a single phase motor but decided it would just add to the idiot arguments already in place. :-)
    ...lew...

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    • #32
      Originally posted by Noitoen View Post
      DC is used because you get a stable and silent magnetic field. AC will always vibrate at line frequency and that's why ac cores on relay coils have a little copper bar one turn short to reduce this vibration.
      Once I was working on a roll up style garage door.
      It was 20 feet tall and 30 feet wide, at a train depot.
      The problem is that it would over run its down position
      and the slats would start to bunch up. Seemingly
      randomly, other times it would stop short and remain
      open a few inches to a foot. Very random closed position
      was the problem statement.
      Anyhow, working on the operator I was up on 4 lifts of
      scaffold, playing with live 480v contactors and 24VAC
      coils. The tally counter that controlled the limit switches
      was worked fine. The cause eluded me for a while.
      Then I figured it out. The copper shaded pole ring
      on the down contactor was broken. Its purpose was
      to mediate the effect of magnetic hysteresis of the steel
      laminated core. If the AC was interrupted on the top peak
      of the wave, the magnetism would take longer to decay,
      the contactor would run the motor longer, and the door
      would close too much. If the AC was interrupted near
      the zero cross, the motor would stop sooner, and the
      door would stop short. Replacing the contactor solved
      the problem and the garage door returned to consistent
      operation.
      I used to think the copper shading pole was also to
      help mitigate the AC humm that is sometimes associated
      with AC coil contactors. But the true reason is to provide
      a consistent timely decay of the magnetic field, which will
      provide a repeatable release time interval of the contactor.
      Normally milliseconds are not important for the use of
      a contactor in service to switch on and off the motor of
      a machine tool. But when used in applications where
      time is utilized to control objects positionally, millisecond
      consistent contact release makes a big deal.

      --Doozer
      DZER

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      • #33
        The reason for a shading coil on both contactor coil and a fan motor is to provide a phase shift in that part of the laminations.
        A contactor has the tendency to drop out as the sine wave transits through zero, the shading pole offers a shifted magnet field, some degrees away from the principle frequency.
        In a shaded pole motor, the shading ring offers the same degree of shift as a start winding and a capacitor.

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        • #34
          Originally posted by MaxHeadRoom View Post
          ...
          A contactor has the tendency to drop out as the sine wave transits through zero, ....
          You might think so, but the hysteretic properties of the steel core prevents this.
          The shading pole winding provides a controlled magnetic limiter, to prevent too much
          magnetism from developing in the core. But it does not completely snub it, or the
          solenoid would not draw in at all.

          -D

          DZER

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          • #35
            One of many example reasons, Internet referenced.

            "Shading Coil's purpose is to provide sizeable phase-shifted magnetic field to keep the contactor on when the main coil flux passes through zero, avoiding unwanted chatter and mechanical destruction of the magnet and power contacts". .

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            • #36
              The reason I was wary of the presence of an electromagnetically acctuated brake attached to the motor is that probably that technology predated VFD's with their programmable braking through the three phase windings, using an additional resistor module.

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              • #37
                Originally posted by MaxHeadRoom View Post
                One of many example reasons, Internet referenced.

                "Shading Coil's purpose is to provide sizeable phase-shifted magnetic field to keep the contactor on when the main coil flux passes through zero, avoiding unwanted chatter and mechanical destruction of the magnet and power contacts". .
                The internet is incorrect.

                -Doozer
                DZER

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                • #38
                  Originally posted by old mart View Post
                  The reason I was wary of the presence of an electromagnetically acctuated brake attached to the motor is that probably that technology predated VFD's with their programmable braking through the three phase windings, using an additional resistor module.
                  These are 2 different things. VFD without resistor can inject a dc voltage on the windings and it will stop the motor almost instantly but then it released. the electromagnetic brake on the other hand will keep it stopped
                  Helder Ferreira
                  Setubal, Portugal

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                  • #39
                    Yes but there might have to be some extra relays to achieve both at the same time, I don't know if a VFD can handle both.

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                    • #40
                      Shading ring.
                      Practical demonstration!!

                      https://www.youtube.com/watch?v=bynC1d8irLI

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                      • #41
                        Max-
                        That is a good video demonstration.
                        I was wrong.
                        It seems there are a few things going on magnetically.
                        Thanks for the good video link.

                        --Doozer
                        DZER

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                        • #42
                          I had a problem once with a MIG welding machine that, when I released the trigger would do everything right like stopping the wire and gas but the main contactor would stay on for a random amount of time up to almost 3 or 4 seconds. This was strange because the relay on the control board that powered the main contactor would disconnect at the right time. Turned out that some old stickey lubricant that someone applied to the core of the contactor would keep the cores stuck together until the spring slowly separated them. This time would vary randomly.
                          Helder Ferreira
                          Setubal, Portugal

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                          • #43
                            If you're worried about what to feed it, American motors like hot dogs and German motors like bratwurst.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #44
                              Originally posted by Doozer View Post

                              The internet is incorrect.

                              -Doozer
                              Gasp! Say it isn't so!

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                              • #45
                                Yes, that was a very informative video. I do find myself wondering how often and why a shading coil would fail or be damaged though. For that demo he likely cut a chunk out of it, but safely buried inside the housing of the contactor I'm not sure how it might come to grief. Back in post #32, above Doozer mentioned that he did find a failed coil. Any hints as to what went wrong in that particular case?

                                Edit : How the $#@!? did my reference to a post in this thread get rewritten as one in a completely different thread? fixed. I hope.
                                Last edited by mickeyf; 05-07-2022, 04:59 PM.
                                "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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